Criteria are sought for the characterization of metal sites in various electron-transfer proteins such as cytochrome-c, cytochrome oxidase, and iron-sulfur proteins. Physical techniques include variable-temperature magnetic susceptibility, variable-temperature electron paramagnetic resonance, infrared, Raman, Fe57 Mossbauer, and electronic absorption spectroscopies. Major emphasis is placed on understanding the fundamental nature of electron transfer between transition metal ions that are well separated. Discrete mixed-valence compounds, e.g., biferrocenes and metal complexes with binucleating ligands, are being studied to give insight into the electronic structure of the iron-sulfur proteins that are known to be mixed valent. Ferric spin-equilibrium complexes are under investigation to understand the factors controlling the rate of spin-flipping in spin-equilibrium cytochromes. Binuclear transition metal complexes with very extended bridging groups are being studied to better appreciate the limiting distances over which interactions can be propagated between two metal ions. Several iron complexes incorporating anions of hydroquinones and semiquinones are being prepared and characterized as they relate to the electron transfer reaction of ubiquinone.